Semiconductor substrate for solid-state image sensing device as well as solid-state image sensing device and method for producing the same

ABSTRACT

There is provided a semiconductor substrate for solid-state image sensing device in which the production cost is lower than that of a gettering method through a carbon ion implantation and problems such as occurrence of particles at a device production step and the like are solved. 
     Silicon substrate contains solid-soluted carbon having a concentration of 1×10 16 -1×10 17  atoms/cm 3  and solid-soluted oxygen having a concentration of 1.4×10 18 -1.6×10 18  atoms/cm 3 .

TECHNICAL FIELD

This invention relates to a semiconductor substrate for a solid-stateimage sensing device as well as a solid-state image sensing device and amethod for producing the same.

RELATED ART

The solid-state image sensing device is produced by forming circuits ona semiconductor substrate made of a silicon single crystal. In thiscase, if a heavy metal is incorporated into the semiconductor substrateas an impurity, the electric properties of the solid-state image sensingdevice are deteriorated remarkably.

As a factor of incorporating the heavy metal as an impurity into thesemiconductor substrate are, mentioned firstly a metal contamination atthe production step of the semiconductor substrate and secondly a heavymetal contamination at the production step of the solid-stateimage-sensing device.

As the former case, there is considered the contamination with heavymetal particles generated from a constituent material of an epitaxialgrowth furnace when an epitaxial layer is grown on a substrate of asilicon single crystal or with heavy metal particles generated by thecorrosion of a metal as a piping material based on the use ofchlorine-based gas. Recently, the metal contamination in the epitaxialgrowth step is attempted to be improved by replacing the constitutionalmaterial of the epitaxial growth furnace with a material having acorrosion resistance, but it is difficult to completely avoid the metalcontamination at the epitaxial growth step.

Therefore, the metal contamination at the epitaxial growth step hashitherto been avoided by forming a gettering layer in the interior ofthe semiconductor substrate, or by using a substrate having a highgettering ability for a heavy metal such as a high concentration boronsubstrate or the like.

On the other hand, in the latter case for the production step of thesolid-state image sensing device, there is a fear of contaminating thesemiconductor substrate with heavy metal at each of ion implantationstep, diffusion step and oxidation heat treatment step among steps ofproducing the device.

In order to avoid the contamination of heavy metal in the vicinity of anactive layer in the device, there is commonly used an intrinsicgettering method wherein oxygen precipitate is formed in thesemiconductor substrate, or an extrinsic gettering method wherein agettering site such as backside damage or the like is formed in a backside of the semiconductor substrate.

In case of the above conventional gettering method, i.e. intrinsicgettering method, however, a multi-stage heat treatment step is requiredfor previously forming the oxygen precipitate in the semiconductorsubstrate, so that there is feared the increase of the production cost.Furthermore, it is required to conduct the heat treatment at a highertemperature for a long time, so that there is feared the new metalcontamination to the semiconductor substrate.

In case of the extrinsic gettering method, since the back side damage orthe like is formed in the back side, particles are generated from theback side at the device production step, which causes drawbacks such asformation of defective device and the like.

In the light of the above problems, Patent Document 1 proposes atechnique wherein a predetermined dose of ions such as carbon isimplanted into one surface of a silicon substrate to form an epitaxiallayer of silicon on such a surface for the purpose of reducing whitedefects generated by a dark current, which exerts on electriccharacteristics of the solid-state image sensing device. According tothis technique, the white defects in the solid-state image sensingdevice are largely reduced as compared with the epitaxial substrateusing the conventional gettering method.

As pointed out in paragraph [0018] of Patent Document 2, in thegettering sink formed by the carbon ion implantation described in PatentDocument 1, when the treating temperature at the device production stepbecomes too high after the formation of the epitaxial layer, thegettering effect by the gettering sink rather lowers. That is, in thegettering sink formed by the carbon ion implantation, there is a limitin the gettering effect. In the technique described in Patent Document2, therefore, it is devised to set an upper limit to the subsequenttreating temperature for sufficiently educing the effect of the buriedgettering sink layer through carbon introduction.

Patent Document 1: JP-A-H06-338507

Patent Document 2: JP-A-2002-353434

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

Since the gettering effect is critical in the gettering sink formed bythe carbon ion implantation, for example, the upper limit is set in thedevice treating temperature after the formation of the epitaxial layer,which results in a restriction in the device production step.

Also, the gettering effect of the gettering sink formed by the carbonion implantation tends to be lowered after the formation of theepitaxial layer, which is difficult to avoid the occurrence of particlesin the device production step, so that it is an important matter to makesatisfactory the gettering effect at the device production step.

It is, therefore, an object of the invention to provide a semiconductorsubstrate for solid-state image sensing device being low in theproduction cost and solving problems such as occurrence of particles andthe like at the device production step as compared with the conventionalgettering method, particularly the gettering method through carbon ionimplantation as well as an advantageous method for producing the same.

Further, it is another object of the invention to provide ahigh-performance solid-state image sensing device providing excellentelectric characteristics by forming a circuit on the above semiconductorsubstrate as well as an advantageous method for producing the same.

Means for Solving Problems

The inventors have made various studies on a means for avoiding thecontamination of heavy metal to the semiconductor substrate withoutincreasing the production cost. As a result of the examination on thegettering method through the carbon ion implantation, it is revealedthat the gettering action through the carbon ion implantation is mainlydependent on an oxide precipitated from disorder (strain) of siliconlattice by the ion implantation through high energy and such a latticedisorder concentrates in a narrow ion-implanted zone and also strainaround the oxide is easily released in a high temperature heat treatmentof, for example, the device production step, and hence the getteringeffect is particularly poor in the heat treatment step of the device.

Now, the action of carbon engaging in the formation of the getteringsink in the silicon substrate is examined in detail, and as a result, ithas been discovered that carbon is solid-soluted in the silicon latticein a silicon-substituting form instead that carbon is forcedlyintroduced by the ion implantation and carbon-oxygen based precipitatesinvolving dislocation (carbon-oxide composite) are elicited from thesubstituted carbon at a high density in, for example, the deviceproduction step and such carbon-oxygen based precipitates provide a highgettering effect. Furthermore, it has been found that such a substitutedcarbon is first introduced by including into a silicon single crystal ata solid-solution state, and as a result, the invention has beenaccomplished.

That is, the summary of the invention is as follows.

(1) A semiconductor substrate for solid-state image sensing device,characterized in that a silicon substrate contains solid-soluted carbonhaving a concentration of 1×10¹⁶-1×10¹⁷ atoms/cm³ and solid-solutedoxygen having a concentration of 1.4×10¹⁸-1.6×10¹⁸ atoms/cm³.

(2) A semiconductor substrate for solid-state image sensing deviceaccording to the item (1), wherein an epitaxial layer of silicon isexistent on the silicon substrate.

(3) A semiconductor substrate for solid-state image sensing deviceaccording to the item (2), wherein silicon oxide layer is existent onthe epitaxial layer.

(4) A semiconductor substrate for solid-state image sensing deviceaccording to the item (3), wherein a silicon nitride layer is existenton the oxide film.

(5) A method for producing a semiconductor substrate for solid-stateimage sensing device, characterized in that silicon crystal ispreviously included with carbon at a solid-soluted concentration of1×10¹⁶-1×10¹⁷ atoms/cm³ and oxygen at a solid-soluted concentration of1.4×10¹⁸-1.6×10¹⁸ atoms/cm³ in the production of a single crystalsilicon substrate.

(6) A method of producing a semiconductor substrate for solid-stateimage sensing device according to the item (5), wherein the singlecrystal silicon substrate is produced by using CZ (Czochralski) methodor MCZ (magnetic field applied Czochralski crystal growth) method.

(7) A solid-state image sensing device comprising a silicon substrateand a buried-type photodiode formed thereon, characterized in thatcarbon-oxide based precipitates having a size of not less than 10 nm areexistent in the silicon substrate at a density of 1×10⁶-1×10⁸precipitates/cm².

Moreover, the term “size” used herein means a diagonal length of theprecipitate in TEM image observed at a section of the silicon substratein a thickness direction thereof, and is expressed by an average valueof the precipitates in the observed field.

(8) A method for producing a solid-state image sensing device,characterized in that a layer required for the formation of a device isformed on a silicon substrate containing solid-soluted carbon of1×10¹⁶-1×10¹⁷ atoms/cm³ and solid-soluted oxygen of 1.4×10¹⁸-1.6×10¹⁸atoms/cm³ in a silicon single crystal and then subjected to a heattreatment to promote oxygen precipitation reaction to thereby form agettering sink through carbon-oxygen based precipitates in the siliconsubstrate.

(9) A method for producing a solid-state image sensing device accordingto the item (8), wherein the heat treatment is a heat treatment in theproduction process of the device.

(10) A method for producing a solid-state image sensing device accordingto the item (8) or (9), wherein carbon-oxygen based precipitates havinga size of not less than 10 nm are precipitated at a density of1×10⁶-1×10⁸ precipitates/cm² at the step of the heat treatment.

Effect of the Invention

In the semiconductor substrate for solid-state image sensing deviceaccording to the invention, carbon-oxygen based precipitates having ahigh gettering ability can be formed by including solid-soluted carboninto CZ crystal or MCZ crystal and utilizing the heat treatment step inthe production step of mounting a device on the semiconductor substrate.

Therefore, the gettering sink can be foamed just beneath the buriedphotodiode over a full thickness of the silicon substrate, so that thediffusion of heavy metal at the device production step is particularlysuppressed to avoid the occurrence of defects in the device, and henceit is possible to provide a high quality solid-state image sensingdevice having good electric characteristics at a low cost.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is schematic view illustrating a production procedure of asemiconductor substrate for solid-state image sensing device and furthera solid-state image sensing device according to the invention.

FIG. 2 is a graph showing a relation between a concentration ofsolid-soluted carbon and a number of carbon-oxygen based precipitates ina silicon substrate.

FIG. 3 is a graph showing a relation between a concentration ofsolid-soluted carbon and a size of a carbon-oxygen based precipitate ina silicon substrate.

FIG. 4 is a graph showing a relation between a solid-soluted carbonconcentration in a silicon substrate and a leak current at pn junctionof photodiode.

FIG. 5 is a schematic view illustrating a production procedure of asolid-state image sensing device.

DESCRIPTION OF REFERENCE SYMBOLS

1 silicon substrate

2 epitaxial layer

3 semiconductor substrate

4 silicon oxide layer

5 silicon nitride layer

6 solid-state image sensing device

BEST MODE FOR CARRYING OUT THE INVENTION

Next, the semiconductor substrate according to the invention will bedescribed in detail with reference to the drawings.

FIG. 1 is a schematic view illustrating a production method of asemiconductor substrate for solid-state image sensing device accordingto the invention. In the illustrated embodiment, heaped polysilicon as astarting material for silicon crystal is first placed in, for example, aquartz crucible and further a proper amount of graphite powder isapplied onto the surface of the polysilicon, and then a CZ crystal addedwith carbon is prepared, for example, according to a Czochralski method(CZ method).

Moreover, the CZ crystal is an address term of crystals inclusive of amagnetic field applied CZ crystal prepared according to the Czochralskimethod.

At this moment, carbon is added at a stage of the starting material anda silicon single crystal is prepared from the carbon added startingmaterial, whereby there is obtained a silicon substrate 1 containingcarbon at a solid-soluted state (see FIG. 1( a)). The thus obtainedsilicon substrate 1 is important to contain solid-soluted carbon havinga concentration of 1×10¹⁶-1×10¹⁷ atoms/cm³ and solid-soluted oxygenhaving a concentration of 1.4×10¹⁸-1.6×10¹⁸ atoms/cm³.

At first, the reason why carbon is included at a solid-soluted state isdue to the fact that carbon is introduced into silicon lattices in formof substituting with silicon. That is, since the atomic radius of carbonis shorter than that of silicon atom, when carbon is coordinated in asubstitution position, a stress field of a crystal is a compressionstress field and interstitial oxygen and impurities are apt to be easilycaught. Therefore, precipitates with oxygen involving dislocation areeasily created from the substitution-positioned carbon, for example, atthe device production step in a high density, whereby a high getteringeffect can be given to the silicon substrate 1.

It is required that the concentration of solid-soluted carbon added iscontrolled to a range of 1×10¹⁶-1×10¹⁷ atoms/cm³. When the concentrationof solid-soluted carbon is less than 1×10¹⁶ atoms/cm³, the promotion offorming the carbon-oxygen based precipitates is not active, so that theformation of the carbon-oxygen based precipitates at a high density cannot be attained.

The results measured on the number of carbon-oxygen based precipitatesby varying the concentration of solid-soluted carbon in the siliconsubstrate are shown in FIG. 2. Moreover, the measurement of theconcentration of solid-soluted carbon is carried out by a Fouriertransformation infrared absorption spectrophotometry (FT-IR). Also, themeasurement of the carbon-oxygen based precipitate number is carried outby observing TEM image at a section of the silicon substrate in athickness direction thereof. As shown in FIG. 2, when the concentrationof solid-soluted carbon is less than 1×10¹⁶ atoms/cm³, the formation ofthe carbon-oxygen based precipitates decreases extremely.

On the other hand, when it exceeds 1×10¹⁷ atoms/cm³, the formation ofthe carbon-oxygen based precipitates is promoted to obtain high densitycarbon-oxygen based precipitates, but the size of the precipitate issuppressed and hence strain around the precipitate strongly tends to beweakened. Therefore, the effect of the strain becomes weak and theeffect for catching the impurity reduces.

The results measured on the size of the carbon-oxygen based precipitateby varying the concentration of solid-soluted carbon in the siliconsubstrate are shown in FIG. 3. Moreover, the size of the carbon-oxygenbased precipitate is measured by observing TEM image at a section of thesilicon substrate in a thickness direction thereof to determine adiagonal line of the precipitate and expressed by an average of themeasured values.

As shown in FIG. 3, when the concentration of solid-soluted carbonexceeds 1×10¹⁷ atoms/cm³, the size of the carbon-oxygen basedprecipitate becomes extremely small.

Further, when it is required that the concentration of solid-solutedoxygen in the silicon substrate 1 is controlled to a range of1.4×10¹⁸-1.6×10¹⁸ atoms/cm³. When the concentration of solid-solutedoxygen is less than 1.4×10¹⁸ atoms/cm³, the formation of thecarbon-oxygen based precipitates is not promoted, and hence the abovehigh density carbon-oxygen based precipitates are not obtained.

On the other hand, when it exceeds 1.6×10¹⁸ atoms/cm³, the size ofoxygen precipitate is reduced and the effect of strain at an interfaceof the precipitate to silicon atom of the matrix is mitigated, and hencethere is a fear of lowering the gettering effect through the strain.

Since the density of the carbon-oxygen based precipitates becomesinvolved in the occurrence of white defects in the solid-state imagesensing device, a relation between the concentration of solid-solutedcarbon and the concentration of solid-soluted oxygen in the siliconsubstrate 1 is examined with respect to a leak current of a photodiodejunction proportional to the number of white defects. The results areshown in FIG. 4.

Moreover, the leak current of the photodiode junction is made asfollows. At first, the silicon substrate is cleaned with a SC-1 cleaningsolution (NH₄OH:H₂O₂:H₂O=1:1:5) and then cleaned with a SC-2 cleaningsolution (HCl:H₂O₂:H₂O=1:1:5). Next, the wafer is wet-oxidized at 1100°C. for 110 minutes to form a field silicon oxide layer of 600 nm inthickness on the surface of the wafer. Thereafter, the silicon oxidelayer is patterned by a photolithography to form a diffusion window, andn⁺ layer is formed thereon by a solid layer diffusion using phosphorusoxychloride (POCl₃). In this case, the condition of phosphorus diffusionis that PSG (phosphorus silicate glass) film is removed by etching at900° C. for 20 minutes and thermal diffusion is conducted at 1000° C.for 60 minutes. The n⁺ layer has a diffusion depth of about 2 μm and aconcentration of 1×10¹⁹/cm³. After the formation of a contact hole, Alcontaining 1.5 mass % of Si is deposited thereon at a thickness of 500nm by sputtering. After the patterning of an electrode, an annealingtreatment is carried out at 450° C. in N₂ atmosphere, and finally anoxide film is removed from a back surface. A pattern having a joint areaof 1.8 mm□ is used.

A leak current at the thus obtained pn junction of the silicon substrateis measured by means of HP4140 (pA) meter by applying a voltage to sucha junction. In this case, it is designed to suppress a p-type surfaceinversion by applying a bias to a guard ring by HP4141B (current voltagesource). As a negative guard ring bias is used −20 V. The measurement isconducted at 20 places of the silicon substrate (wafer), and an averagethereof is a leak current.

As shown in FIG. 4, when the concentration of solid-soluted carbon iswithin a range of 1×10¹⁶-1×10¹⁷ atoms/cm³, the leak current at pnjunction decreases. Further, noting the concentration of solid-solutedoxygen, when the concentration of solid-soluted oxygen is less than1.4×10¹⁸ atoms/cm³ and becomes 1.3×10¹⁸ atoms/cm³, it is confirmed thatthe leak current at pn junction largely decreases even at anyconcentrations of solid-soluted carbon. This tendency on theconcentration of solid-soluted oxygen is recognized even in FIGS. 2 and3 in a similar fashion.

On the other hand, when the concentration of solid-soluted oxygenexceeds 1.6×10¹⁸ atoms/cm³, or when it becomes 1.7×10¹⁸ atoms/cm³exceeding 1.6×10¹⁸ atoms/cm³ as shown in FIG. 3, the size of thecarbon-oxygen based precipitate decreases. This decrease of the sizeleads to the mitigation of the strain effect at the interface betweensilicon atom of the matrix and the precipitate, so that it is feared tolower the gettering effect through the strain.

Next, the silicon substrate 1 being the carbon added CZ crystal issubjected to RCA cleaning with, for example, a combination of SC1 andSC2 in order to grow an epitaxial layer after the surface mirroring ofthe substrate. Thereafter, the substrate is placed in an epitaxialgrowth furnace to grow an epitaxial layer 2 having a predeterminedthickness (see FIG. 1( b)).

Moreover, various CVD methods (chemical vapor deposition method) can beused in the growth of the epitaxial layer 2.

At this moment, the thickness of the epitaxial layer 2 is preferable tobe a range of 2-9 μm in view that the spectral response characteristicsof the solid-state image sensing device are improved.

A semiconductor substrate 3 provided with the above epitaxial layer 2 issupplied to the following device production step after an silicon oxidelayer 4 and further a silicon nitride layer 5 are formed on theepitaxial layer 2, if necessary. At this step, a buried type photodiodeis formed in the epitaxial layer 2 to obtain a solid-state image sensingdevice 6.

Moreover, the thicknesses of the silicon oxide layer 4 and the siliconnitride layer 5 are preferable to be 50-100 nm in the silicon oxidelayer 4 and 1.0-2.0 μm in the silicon nitride layer 5, concretelypolysilicon gate film 5 in the solid-state image sensing device from therestrictions in the designing of an operating voltage of a chargetransfer transistor.

The silicon substrate 1 in the semiconductor substrate 3 supplied to thedevice production step is the CZ crystal containing solid-solutedcarbon, but oxygen precipitating nucleus or oxygen precipitate formed inthe crystal growth is shrunk by the heat treatment in the epitaxialgrowth, so that the actualized oxygen precipitates are not existent inthe silicon substrate 1 at the stage of the semiconductor substrate 3.

Therefore, in order to ensure a gettering sink for gettering heavymetal, it is required that the substrate is subjected to a lowtemperature heat treatment of, preferably, about 600° C.-700° C. afterthe growth of the epitaxial layer to separate out carbon-oxygen basedprecipitates 7 from the substitution-positioned carbon (see FIG. 1( c)).

In the device production step or the general step of producing thesolid-state image sensing device, the initial stage thereof is common toinclude a heat treatment step of about 600° C.-700° C., so that it ispossible to grow and form the carbon-oxygen based precipitates by usingthe semiconductor substrate 3 containing the solid-soluted carbon as asubstrate for solid-state image sensing device through the deviceproduction step.

In the invention, the “carbon-oxygen based precipitate” means aprecipitate of carbon-oxygen composite (cluster) containing carbon.

The carbon-oxygen based precipitates are spontaneously separated outover the whole of the silicon substrate 1 through the initial stage ofthe device production step by using the semiconductor substrate 3containing the solid-soluted carbon, so that the gettering sink having ahigh gettering ability against metal contamination at the deviceproduction step can be formed over a full thickness of the siliconsubstrate 1 just beneath the epitaxial layer. Therefore, the getteringin a region close to the epitaxial layer can be realized.

In order to realize this gettering, the carbon-oxygen based precipitatesare important to have a size of not less than 10 nm and to be existentin the silicon substrate at a density of 1×10⁶-1×10⁸ precipitates/cm².

When the size of the carbon-oxygen based precipitates is not less than10 nm, the probability of catching (gettering) impurities between thelattices (e.g. heavy metal and so on) is increased by utilizing thestrain effect generated at the interface between silicon atom of thematrix and the oxygen precipitate.

Also, the density of the carbon-oxygen based precipitates having a sizeof not less than 10 nm is within a range of 1×10⁶-1×10⁸ precipitates/cm²because the catching (gettering) of heavy metal in the silicon crystalis dependent on the strain generated at the interface between siliconatom of the matrix and the oxygen precipitate and an interface statedensity (volume density).

Moreover, as the above device production step can be adopted the generalproduction step for the solid-state image sensor. An example of such astep is shown in FIG. 5 for CCD device, but it is not particularlylimited to FIG. 5.

In the device production step, there is first provided a semiconductorsubstrate 3 wherein an n-type epitaxial layer 2 is formed on a siliconsubstrate 1 shown in FIG. 5( a), and then a first p-type well region 11is formed at a predetermined position of the epitaxial layer 2 as shownin FIG. 5( b). Thereafter, as shown in FIG. 5( c), a gate insulatingfilm 12 is formed on the surface, while n-type and p-type impurities areselectively implanted into the interior of the first p-type well region11 by ion implantation to form n-type transfer channel region 13, p-typechannel stop region 14 and second p-type well region 15 constituting avertical transfer resistor, respectively.

Then, as shown in FIG. 5( d), a transfer electrode 16 is formed on thesurface of the gate insulating film 12 at a predetermined position.Thereafter, as shown in FIG. 5( e), n-type and p-type impurities areselectively implanted between the n-type transfer channel region 13 andthe second p-type well region 15 to form a photodiode 19 comprised of alaminate of p-type positive charge storing region 17 and n-type impuritydiffusing region 18.

Further, as shown in FIG. 5( f), an interlayer insulating film 20 isformed on the surface, and then a light shielding film 21 is formed onthe surface of the interlayer insulating film 20 other than a portionjust above the photodiode 19, whereby a solid-state image sensing device10 can be produced.

At the above device production step, it is usual to conduct a heattreatment of about 600-1000° C. in, for example, the formation of gateoxide film, separation of element and formation of polysilicon gateelectrode. In this heat treatment can be attempted the aboveprecipitation of the carbon-oxygen based precipitates 7, which can actas a gettering sink in the subsequent steps.

EXAMPLES

A heaped polysilicon as a starting material for silicon crystal isplaced in a quartz crucible and a proper amount of graphite powder isapplied onto the surface of the polysilicon, and then a CZ crystal addedwith carbon is prepared according to a Czochralski method (CZ method).Concentrations of solid-soluted carbon and solid-soluted oxygen in asilicon substrate cut out as a wafer from the CZ crystal are shown inTable 1. Then, the thus obtained silicon substrate is subjected to asurface contamination (contaminant: Fe, Cu, Ni and contaminationconcentration: 1×10¹³ atoms/cm²) by a spin coating method, and furthersubjected to a heat treatment under temperature conditions correspondingto a heat treatment in the production of a solid-state image sensingdevice.

With respect to a gettering ability of the silicon substrate, acontamination concentration of metals on the surface of the siliconsubstrate is measured by an atomic spectrophotometry and a getteringefficiency is determined according to the following equation.Account Gettering efficiency=(surface contamination concentration afterheat treatment)/(initial surface contamination concentration)×100(%)

The results are also shown in Table 1 as gettering efficiency of notless than 90%: ⊚, less than 90% but not less than 80%: ◯, less than 80%but not less than 50%: Δ and less than 50%: χ, from which it isunderstood that the semiconductor substrate according to the inventionhas a sufficient durability to the contamination of heavy metal at theproduction step of the solid-state image sensing device.

TABLE 1 Concentration of solid-soluted oxygen (×10¹⁸ atoms/cm³) 1 × 10¹³1 × 10¹⁴ 1 × 10¹⁵ 1 × 10¹⁶ 1 × 10¹⁷ Concentration 0.5 × 10¹⁶ X X X X Xof solid- 1.0 × 10¹⁶ X Δ Δ Δ Δ soluted 2.0 × 10¹⁶ X Δ Δ Δ Δ carbon 3.0 ×10¹⁶ ◯ ◯ ◯ ◯ ◯ (atoms/cm³) 5.0 × 10¹⁶ ⊚ ⊚ ⊚ ⊚ ⊚ 7.0 × 10¹⁶ ⊚ ⊚ ⊚ ⊚ ⊚10.0 × 10¹⁶  X Δ Δ Δ Δ 15.0 × 10¹⁶  X X X X X

Then, as shown in FIG. 1( b), the surface of the silicon substrate 1 issubjected to a mirror polishing, RCA-cleaned with a combination of SC-1and SC-2, and placed in an epitaxial growth furnace to form an epitaxiallayer 2 having a thickness of 4.5 μm by CVD method. Moreover, the CVDmethod is conducted by using SiHCl₃ (trichlorosilane) and SiH₄(monosilane) as a starting gas.

A solid-state image sensing device is produced by preparing CMOS deviceon the thus formed semiconductor substrate 3 provided with the epitaxiallayer 2 according to the procedures shown in FIG. 5. In the step offorming a gate oxide film at the device production step (FIG. 5( c)),the number and size of carbon-oxygen based precipitates in the siliconsubstrate 1 through the heat treatment of 700° C. are investigated. Theresults are shown in Tables 2-6.

With respect to the thus obtained solid-state image sensing device, adark, backward leak current of PN junction diode is investigated. Theresults are also shown in Tables 2-6 as leak current of less than 80arb. Unit: ⊚, not less than 80 arb. Unit but less than 130 arb. Unit: ◯,not less than 130 arb. Unit but less than 160 arb. Unit: Δ and not lessthan 160 arb. Unit: χ, from which it is understood that the leak currentis suppressed in the solid-state image sensor using the semiconductorsubstrate according to the invention.

TABLE 2 Concentration of solid-soluted oxygen: 1.3 × 10¹⁸ (atoms/cm³)Number of Size of Concentration carbon-oxygen carbon- of solid- basedoxygen based soluted carbon precipitates precipitate No (atoms/cm³)(precipitates/cm²) (nm) Leak current 1-1 0.5 × 10¹⁶ 6.0 × 10⁵ 350 X 1-21.0 × 10¹⁶ 9.0 × 10⁵ 310 X 1-3 2.0 × 10¹⁶ 3.0 × 10⁶ 250 X 1-4 3.0 × 10¹⁶6.0 × 10⁶ 200 ◯ 1-5 4.0 × 10¹⁶ 7.0 × 10⁶ 170 ⊚ 1-6 5.0 × 10¹⁶ 1.1 × 10⁷150 ⊚ 1-7 6.0 × 10¹⁶ 1.7 × 10⁷ 130 ⊚ 1-8 7.0 × 10¹⁶ 1.8 × 10⁷ 125 ⊚ 1-98.0 × 10¹⁶ 2.5 × 10⁷ 75 ◯ 1-10 9.0 × 10¹⁶ 2.7 × 10⁷ 60 X 1-11 10.0 ×10¹⁶  3.0 × 10⁷ 50 X 1-12 15.0 × 10¹⁶  2.0 × 10⁸ 30 X

TABLE 3 Concentration of solid-soluted oxygen: 1.4 × 10¹⁸ (atoms/cm³)Number of Size of Concentration carbon-oxygen carbon- of solid- basedoxygen based soluted carbon precipitates precipitate No (atoms/cm³)(precipitates/cm²) (nm) Leak current 2-1 0.5 × 10¹⁶ 1.5 × 10⁶ 310 X 2-21.0 × 10¹⁶ 2.0 × 10⁶ 290 Δ 2-3 2.0 × 10¹⁶ 7.0 × 10⁶ 230 Δ 2-4 3.0 × 10¹⁶1.0 × 10⁷ 180 ◯ 2-5 4.0 × 10¹⁶ 1.5 × 10⁷ 160 ⊚ 2-6 5.0 × 10¹⁶ 2.0 × 10⁷130 ⊚ 2-7 6.0 × 10¹⁶ 3.0 × 10⁷ 120 ⊚ 2-8 7.0 × 10¹⁶ 4.0 × 10⁷ 100 ⊚ 2-98.0 × 10¹⁶ 4.3 × 10⁷ 75 ⊚ 2-10 9.0 × 10¹⁶ 4.6 × 10⁷ 55 Δ 2-11 10.0 ×10¹⁶  5.0 × 10⁷ 30 Δ 2-12 15.0 × 10¹⁶  3.0 × 10⁸ 25 X

TABLE 4 Concentration of solid-soluted oxygen: 1.5 × 10¹⁸ (atoms/cm³)Number of Size of Concentration carbon-oxygen carbon- of solid- basedoxygen based soluted carbon precipitates precipitate No (atoms/cm³)(precipitates/cm²) (nm) Leak current 3-1 0.5 × 10¹⁶ 2.0 × 10⁶ 280 X 3-21.0 × 10¹⁶ 3.0 × 10⁶ 250 Δ 3-3 2.0 × 10¹⁶ 9.0 × 10⁶ 225 Δ 3-4 3.0 × 10¹⁶1.5 × 10⁷ 175 ◯ 3-5 4.0 × 10¹⁶ 2.5 × 10⁷ 145 ⊚ 3-6 5.0 × 10¹⁶ 3.0 × 10⁷105 ⊚ 3-7 6.0 × 10¹⁶ 5.0 × 10⁷ 95 ⊚ 3-8 7.0 × 10¹⁶ 8.0 × 10⁷ 85 ⊚ 3-98.0 × 10¹⁶ 9.0 × 10⁷ 75 ⊚ 3-10 9.0 × 10¹⁶ 9.5 × 10⁷ 50 Δ 3-11 10.0 ×10¹⁶  1.0 × 10⁸ 25 Δ 3-12 15.0 × 10¹⁶  7.0 × 10⁹ 20 X

TABLE 5 Concentration of solid-soluted oxygen: 1.6 × 10¹⁸ (atoms/cm³)Number of Size of Concentration carbon-oxygen carbon- of solid- basedoxygen based soluted carbon precipitates precipitate No (atoms/cm³)(precipitates/cm²) (nm) Leak current 4-1 0.5 × 10¹⁶ 4.0 × 10⁶ 250 X 4-21.0 × 10¹⁶ 5.0 × 10⁶ 230 Δ 4-3 2.0 × 10¹⁶ 1.5 × 10⁷ 190 Δ 4-4 3.0 × 10¹⁶2.0 × 10⁷ 160 ◯ 4-5 4.0 × 10¹⁶ 4.0 × 10⁷ 130 ⊚ 4-6 5.0 × 10¹⁶ 7.0 × 10⁷90 ⊚ 4-7 6.0 × 10¹⁶ 9.0 × 10⁷ 85 ⊚ 4-8 7.0 × 10¹⁶ 1.5 × 10⁸ 80 ⊚ 4-9 8.0× 10¹⁶ 1.7 × 10⁸ 75 ⊚ 4-10 9.0 × 10¹⁶ 1.9 × 10⁸ 60 Δ 4-11 10.0 × 10¹⁶ 2.0 × 10⁸ 15 Δ 4-12 15.0 × 10¹⁶  1.0 × 10⁹ 7 X

TABLE 6 Concentration of solid-soluted oxygen: 1.7 × 10¹⁸ (atoms/cm³)Number of Size of Concentration carbon-oxygen carbon- of solid- basedoxygen based soluted carbon precipitates precipitate No (atoms/cm³)(precipitates/cm²) (nm) Leak current 5-1 0.5 × 10¹⁶ 2.0 × 10⁷ 150 X 5-21.0 × 10¹⁶ 2.5 × 10⁷ 125 Δ 5-3 2.0 × 10¹⁶ 8.0 × 10⁷ 110 Δ 5-4 3.0 × 10¹⁶1.5 × 10⁸ 80 ◯ 5-5 4.0 × 10¹⁶ 2.0 × 10⁸ 75 ⊚ 5-6 5.0 × 10¹⁶ 3.0 × 10⁸ 50⊚ 5-7 6.0 × 10¹⁶ 5.0 × 10⁸ 47 ⊚ 5-8 7.0 × 10¹⁶ 7.0 × 10⁹ 45 ⊚ 5-9 8.0 ×10¹⁶ 7.3 × 10⁹ 30 ⊚ 5-10 9.0 × 10¹⁶ 7.8 × 10⁹ 20 Δ 5-11 10.0 × 10¹⁶  8.0× 10⁸ 10 Δ 5-12 15.0 × 10¹⁶   6.0 × 10¹⁰ 5 X

1. A semiconductor substrate for a solid-state image sensor, comprising:an epitaxial layer of silicon, existent on a silicon substrate; and anoxide film existent on the epitaxial layer; wherein the siliconsubstrate contains solid-soluted carbon having a concentration of3×10¹⁶-8×10¹⁶ atoms/cm³ and solid-soluted oxygen having a concentrationof 1.4×10¹⁸-1.6×10¹⁸ atoms/cm³; wherein carbon-oxygen based precipitateshaving a size in the range of 75 nm to 180 nm (inclusive of 75 nm and180 nm) are existent in the silicon substrate at a density of1×10⁷-1×10⁸ precipitates/cm².
 2. A semiconductor substrate forsolid-state image sensing device according to claim 1, wherein a siliconnitride film is existent on the silicon oxide film.
 3. A method forproducing a semiconductor substrate for solid-state image sensingdevice, comprising: growing an epitaxial layer of silicon on a siliconsubstrate produced from a single silicon crystal containingsolid-soluted carbon at a concentration of 3×10¹⁶-8×10¹⁶ atoms/cm³ andsolid-soluted oxygen at a concentration of 1.4×10¹⁸-1.6×10¹⁸ atoms/cm³;growing an oxide film on the epitaxial layer; subjecting the resultingsilicon substrate to a low temperature heat treatment of 600° C. to 700°C. such that carbon-oxygen based precipitates having a size in the rangeof 75 nm to 180 nm (inclusive of 75 nm and 180 nm) are existent in thesilicon substrate at a density of 1×10⁷-1×10⁸ precipitates/cm².
 4. Amethod of producing a semiconductor substrate for solid-state imagesensing device according to claim 3, wherein the single crystal siliconsubstrate is produced by using CZ (Czochralski) method or MCZ (magneticfield applied Czochralski crystal growth) method.
 5. A solid-state imagesensing device comprising: a silicon substrate; and a buried-typephotodiode formed thereon; wherein carbon-oxygen based precipitateshaving a size in the range of 75 nm to 180 nm (inclusive of 75 nm and180 nm) are existent in the silicon substrate at a density of1×10⁷-1×10⁸ precipitates/cm².